Magnetoresistive (MR) heads are often used for information sensing in magnetic data storage devices. A MR head typically includes an MR element that changes in resistance in correspondence with changing magnetic flux from data patterns on an adjacent magnetic disk surface. A measurement of changes of the resistance of the MR element is used detect binary data stored on the magnetic disk.
In ultrahigh-density magnetic disk drives, correspondingly high sensitivity and stability heads are needed to achieve desired performance. Tunneling magnetoresistive (TMR) are now being used to provide such performance.
A tunneling magnetoresistive (TMR) head generally employs two ferromagnetic layers separated by a thin insulator. Changes of the relative magnetic orientation of the two magnetic layers result in changes of the resistance of a tunneling current through the thin insulator. This is also known as tunneling magnetoresistance (TMR) effect. In TMR heads, typically, a bias voltage, or a bias current having a constant value, is applied across the TMR head, and the change in resistance of the TMR head is measured by sensing a change in the output current or voltage. The changes in the output are generally amplified by a read amplifier circuit.
It is nominally desirable to use a TMR head with a read amplifier in high performance disk drives. However, the impedance of a TMR head generally varies from 300 ohms to 1000 ohms, and prior art read amplifiers typically have a relatively high impedance, for example, as does the read amplifier disclosed in U.S. Patent Publication No. 2002/0176191A1. Generally, the input capacitance of prior art read amplifier and the high impedance of a TMR head form a RC integrator with a relatively large time constant and a relatively low cut-off at high frequencies. As a consequences the read bandwidth can be as low as 100 MHz, instead of at least 1 GHz as required for contemporary disk drives.
As a possible solution, a low impedance amplifier to the type disclosed in U.S. Pat. No. 6,667,842B2 might be used with a TMR head. However, that possible solution is not viable, particularly for drives with perpendicular recording is that the amplifier is not DC coupled due to the decoupling capacitors in the feedback circuits between the collectors and bases of its input transistors. As a result, the DC component of the recorded signal which is important for perpendicular recording, is lost in the read process.
Accordingly, there is a need for a low impedance read amplifier system that is capable of working with high-impedance TMR heads, with high read frequency.